Adhesives and methods of making the same

ABSTRACT

Embodiments of this invention relate to adhesives, and more particularly to biomimetic heteropolymer adhesive compositions. Certain embodiments relate to biomimetic terpolymer adhesive compositions including dopamine methacrylamide, 3,4-dihydroxyphenylalanine, or 3,4-dihydroxy styrene, mimicking moieties found in marine mussel adhesive proteins. In some embodiments, elastic moduli of the adhesives are preferably selected to match the elastic moduli of the substrates to minimize stress concentrations, to increase the ductility of the adhesive-substrate system, or both.

This application claims the benefit of U.S. provisional patentapplication Ser. No. 62/186,369, filed 30 Jun. 2015, for ADHESIVES ANDMETHODS OF MAKING THE SAME, incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of this invention relate to adhesives, and more particularlyto biomimetic heteropolymer adhesive compositions. Certain embodimentsrelate to biomimetic terpolymer adhesive compositions including dopaminemethacrylamide, 3,4-dihydroxyphenylalanine, or 3,4-dihydroxystyrene,mimicking moieties found in marine mussel adhesive proteins. In someembodiments, elastic moduli of the adhesives are preferably selected tomatch the elastic moduli of the substrates to minimize stressconcentrations, to increase the ductility of the adhesive-substratesystem, or both.

BACKGROUND

Adhesives with different chemistries and varying forms have foundwidespread use in the automotive, aerospace, construction, andbiomedical industries. The polymers most often used in adhesives areepoxy, polyurethane, or acrylics. Due to the wide range of uses ofadhesives, optimal bonding is needed for a variety of substrates, jointgeometries, and applications. When designing an adhesive, the elasticmodulus (i.e., stiffness) of both the glue and the substrates beingjoined must be considered. If the elastic modulus of these materialsdiffers and the joint is subjected to mechanical load, the mismatch instiffness generates interfacial stresses that can bring about bondfailure. There exists a need for adhesives suitable for joiningsubstrates of varying stiffness.

SUMMARY

High performance adhesives require mechanical properties tuned todemands of the surroundings. A mismatch in stiffness between substrateand adhesive leads to stress concentrations and fracture when thebonding is subjected to mechanical load. Balancing material strengthversus ductility, as well as considering the relationship betweenadhesive elastic modulus and substrate elastic modulus, will createstronger joints. Mechanical properties of biomimetic heteropolymeradhesives are tailored by controlling the amount of stiffening monomersand softening monomers added to a cross-linking monomer in theheteropolymer. In some embodiments, elastic moduli of the adhesives arepreferably selected to match the elastic moduli of the substrates tominimize stress concentrations, to increase the ductility of theadhesive-substrate system, or both.

In some embodiments, an adhesive composition comprises a heteropolymerincluding one of dopamine methacrylamide or 3,4-dihydroxyphenylalanineor 3,4-dihydroxystyrene, and at least one of methyl methacrylate,styrene, and poly(ethylene glycol) methyl ether methacrylate.

In some embodiments, an adhesive composition comprises a heteropolymerincluding a plurality of monomers including a dopamine moiety and aplurality of monomers including an acrylate moiety.

In some embodiments, a biomimetic polymer adhesive comprises thefollowing components: cross-linking monomer in a proportion of about 28%to about 36% by mole percentage, stiffening monomer in a proportion of0% to about 65% by mole percentage, and softening monomer in aproportion of about 0% to about 72% by mole percentage.

In some embodiments, a method of adhering comprises selecting a pair ofsubstrates to be adhered, determining an elastic modulus of eachsubstrate, and adhering the substrates using a heteropolymer adhesiveincluding a plurality of cross-linking monomers and a plurality ofstiffening monomers, wherein, if the elastic modulus of each of thesubstrates is greater than about 1 GPa, the heteropolymer adhesivefurther includes a plurality of softening monomers.

This summary is provided to introduce a selection of the concepts thatare described in further detail in the detailed description and drawingscontained herein. This summary is not intended to identify any primaryor essential features of the claimed subject matter. Some or all of thedescribed features may be present in the corresponding independent ordependent claims, but should not be construed to be a limitation unlessexpressly recited in a particular claim. Each embodiment describedherein is not necessarily intended to address every object describedherein, and each embodiment does not necessarily include each featuredescribed. Other forms, embodiments, objects, advantages, benefits,features, and aspects of the present invention will become apparent toone of skill in the art from the detailed description and drawingscontained herein. Moreover, the various apparatuses and methodsdescribed in this summary section, as well as elsewhere in thisapplication, can be expressed as a large number of differentcombinations and subcombinations. All such useful, novel, and inventivecombinations and subcombinations are contemplated herein, it beingrecognized that the explicit expression of each of these combinations isunnecessary.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an image displaying flexible substrates, namely, human skin,joined by an adhesive.

FIG. 1B is an image displaying stiff substrates, namely, metalcylinders, joined by an adhesive.

FIG. 1C is an image displaying a flexible substrate, namely, apolyurethane cylinder, joined by an adhesive to a stiff substrate,namely, a metal cylinder.

FIG. 2 is a graph depicting lap shear adhesion (line with circles) andbutt tensile adhesion (line squares) of heteropolymers bonded toaluminum substrates. The heteropolymers each contain approximately 33mole percent dopamine methacrylamide and varying percentages of methylmethacrylate versus poly(ethylene glycol) methyl ether methacrylate. TheY-axis indicates the adhesion strength measured in megapascals. TheX-axis indicates the mole percent poly(ethylene glycol) methyl ethermethacrylate in the heteropolymer, the compositions of eachheteropolymers being listed in Table 1.

FIG. 3A is a graph depicting force versus extension curves from lapshear adhesion testing of heteropolymers bonded to aluminum substrates.The heteropolymers each contain approximately 33 mole percent dopaminemethacrylamide and varying percentages of poly(ethylene glycol) methylether methacrylate (PEG), the remaining percentage being methylmethacrylate. The compositions of each heteropolymer are listed in Table2. The Y-axis indicates the force measured in Newtons and the X-axisindicates the length of extension measured in millimeters.

FIG. 3B is a graph depicting stress versus strain when adhesiveheteropolymers were cast into thin films and tested in tension. Theheteropolymers contain approximately 33 mole percent dopaminemethacrylamide and varying percentages of poly(ethylene glycol) methylether methacrylate (PEG), the remaining percentage being methylmethacrylate. The compositions of each heteropolymer are listed in Table2. The Y-axis indicates the stress measured in megapascals and theX-axis indicates the strain measured in percentage ((finallength−initial length)/initial length)×100.

FIG. 4A is a set of three images depicting the flexibility ofpolyurethane substrates with Shore hardness 75 D (top panel), 80 A(middle panel) and 40 A (bottom panel).

FIG. 4B is a graph depicting butt tensile adhesion of heteropolymeradhesives to different substrates with varying stiffness. Theheteropolymer adhesives contain approximately 33 mole percent dopaminemethacrylamide and varying percentages of methyl methacrylate versuspoly(ethylene glycol) methyl ether methacrylate. The compositions ofeach heteropolymer are listed in Table 2. The Y-axis indicates theadhesion strength measured in megapascals. The X-axis indicates the molepercent poly(ethylene glycol) methyl ether methacrylate in theheteropolymer. The substrates are Shore hardness 40 A polyurethane (linewith black squares), Shore hardness 80 A polyurethane (line with opensquares), Shore hardness 75 D polyurethane (line with grey triangles),poly(vinyl chloride) (PVC) (line with grey circles), and poly(methylmethacrylate) (PMMA) (line with open circles).

FIG. 5 is a ¹H NMR spectrum in d⁶-DMSO of poly{[dopaminemethacrylamide]-co-[methyl methacrylate]-co-[poly(ethylene glycol)methyl ether methacrylate]} terpolymer. Assignments are: δ 0-2.3 ppm(broad, polymer backbone), 3.2 ppm (broad, —OCH₃ from poly(ethyleneglycol methyl ether methacrylate), 3,4-3.7 (broad, —OCH₂CH₂ frompoly(ethylene glycol methyl ether methacrylate and —OCH₃ from methylmethacrylate), 3.8-4.2 ppm (—OCH₂ from poly(ethylene glycol methyl ethermethacrylate), 6.2-6.7 ppm (broad, aromatic), and 8.5-8.8 ppm (broad,hydroxyl).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to selected embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended; any alterations andfurther modifications of the described or illustrated embodiments, andany further applications of the principles of the invention asillustrated herein are contemplated as would normally occur to oneskilled in the art to which the invention relates. At least oneembodiment of the invention is shown in great detail, although it willbe apparent to those skilled in the relevant art that some features orsome combinations of features may not be shown for the sake of clarity.

Any reference to “invention” within this document herein is a referenceto an embodiment of a family of inventions, with no single embodimentincluding features that are necessarily included in all embodiments,unless otherwise stated. Further, although there may be references to“advantages” provided by some embodiments of the present invention, itis understood that other embodiments may not include those sameadvantages, or may include different advantages. Any advantagesdescribed herein are not to be construed as limiting to any of theclaims.

Specific quantities (spatial dimensions, angles, dimensionlessparameters, etc.) may be used explicitly or implicitly herein, suchspecific quantities are presented as examples and are approximate valuesunless otherwise indicated. Discussions pertaining to specificcompositions of matter are presented as examples and do not limit theapplicability of other compositions of matter, especially othercompositions of matter with similar properties, unless otherwiseindicated.

When designing an adhesive, the elastic modulus (i.e., stiffness) ofboth the glue and the substrates being joined must be considered. If theelastic modulus of these materials differs and the joint is subjected tomechanical load, the mismatch in stiffness generates interfacialstresses that can bring about bond failure. FIGS. 1A, 1B and 1C arephotographs displaying adhesives used to join substrates of varyingstiffness. In order to maintain bond integrity, a lower modulus adhesiveshould be used for bonding flexible substrates such as rubber or skin(FIG. 1A). A high modulus adhesive is preferred for stiff substratesincluding metal and hard bone (FIG. 1B). This materials designconsideration becomes more problematic when adhering dissimilarsubstrates of varying stiffness (FIG. 1C). The moduli of the adhesivesshould be chosen to match the moduli of the substrates in order tominimize stress concentrations.

Several analytical models have shown that as the adhesive modulusbecomes increasingly different from the modulus of the substrates beingglued together, stress concentrations in the joint become morepronounced. Generally, improved bond strengths were obtained when thelower modulus adhesives and substrates were paired together. Likewise,higher modulus adhesives performed better on stiffer substrates. Anexamination of bonding similar and dissimilar substrates, found that,due to the mismatch in mechanical properties, the bi-material systemsoften had lower strengths compared to the same-material system.

Tuning a polymeric adhesive to specific moduli, thereby matching themoduli of the substrates, provides an ability to “dial in” polymermoduli for specific substrates to help solve problems such asrubber-to-metal bonding in motor mounts for the automotive industry,tendon-to-bone joinery for biomedical applications, and construction ofaerospace vehicles. An ability to match the adhesive and substratemoduli will improve joint performance. Also important are strength andductility of adhesive materials. In lap shear bond configurations, thepoints of highest stress are at the edges. Use of brittle adhesivesmakes this stress concentration even more pronounced. Stiff adhesiveswith low ductility (i.e., percent elongation) foster crack propagation.A more flexible, ductile adhesive can undergo plastic deformation andextend to high elongation percentages, allowing the mechanical load todistribute throughout the joint in a more uniform manner. However, aproblem emerges in that material ductility gains are typicallyincorporated at the expense of material strength. A highly ductileadhesive is often a weak material and can only sustain minimal loads.Studies with different adhesive chemistries and hybrid designs that useseparated segments of stiff and flexible glues along the bondline haveshown that ductility does influence joint strength, yet the optimumbalance between strength and ductility often remains unclear.

Studies have been carried out with a simplified mimic of mussel adhesiveproteins, poly[(3,4-dihydroxystyrene)-co-styrene]. By exploring changesto the polymer composition, molecular weight, and formulation (e.g.,polymer concentration, cure time, cure temperature, and adding fillers)we have been able to, in some cases, obtain strengths higher thancommercial products such as “Super Glue.” Modifications to thisstyrene-based system have also provided insight into a more compliantadhesive. Oligo(ethylene glycol) chains were added topoly[(3,4-dihydroxystyrene)-co-styrene] to yieldpoly{[3,4-dihydroxystyrene]-co-[4-vinylbenzyl {methyltetra(ethyleneglycol)}]-co-styrene}. When working with this system, we noticed thatincreasing oligo(ethylene glycol) content resulted in less brittlepolymers. Although we may have been able to work with this polymer togain insight into how to tune an adhesive to obtain different mechanicalproperties, the six step synthesis became cumbersome. We have now turnedto a methacrylate-based biomimetic polymer that can display a widevariety of moduli and be made in only two steps.

An ideal adhesive should have an optimum balance between strength andductility, while also displaying a modulus similar to the substratesbeing bonded together. To the best of our knowledge, there are nopublished systematic data examining the interplay between strength,ductility, and modulus, within an adhesive material in relation to themodulus of the substrates being bonded together. Gaining insights on theinterplay between these critical parameters will allow us to create thestrongest adhesive joints possible.

Here, a family of biomimetic terpolymers were synthesized with varyingamounts of methyl methacrylate and poly(ethylene glycol) methyl ethermethacrylate (PEG), while keeping dopamine methacrylamide constant.Incorporation of the methyl methacrylate monomer brought about stiffnesswhereas the PEG chains appended to an acrylate monomer promotedflexibility. The adhesive monomer was dopamine methacrylamiderepresenting the DOPA of mussel proteins. Each of these polymers wasthen adhered to substrates of differing stiffness. Starting from abiomimetic design, we were able to systematically modify an adhesive andidentify regions of maximum bonding performance. In the end, the highestperformance bonds were found where the strength and ductility of theadhesive were complementary.

A family of adhesive terpolymers with the structure shown in Scheme 1was obtained via radical polymerization. The dopamine methacrylamideadhesive monomer was prepared in bulk (˜10 grams) following a publishedprocedure. When incorporating this compound into the backbone, ˜33 molepercent was targeted given that an analogous composition showed thehighest adhesion with poly[(3,4-dihydroxystyrene)-co-styrene]. The othermonomers used here were commercially available poly(ethylene glycol)methyl ether methacrylate (Mn˜300 g mol⁻¹) and methyl methacrylate.Molecular weights above ˜5,000 g mol⁻¹ are preferred for obtaining bulkadhesion. Consequently, molecular weights above this value weretargeted.

Polymer characterization was carried out with proton nuclear magneticresonance (¹H NMR) spectroscopy and gel permeation chromatography (GPC)(Table 1). The final percentage of monomers in the backbone followed theinitial feeds. The dopamine methacrylamide content ranged from ˜28-36mole percent, with methyl methacrylate and poly(ethylene glycol) methylether methacrylate (PEG) varying intentionally from ˜0-70 mole percent.The number-average molecular weights (Mn) ranged from ˜6,000 g mol⁻¹ to˜25,000 gmol⁻¹ with polydispersity indices (PDIs) of 1.3-2.0 (Table 1).Thermal characterization using differential scanning calorimetry (DSC)was met with limited success. The glass transition temperatures (Tg) forthe 100% poly(methyl methacrylate) and 100% oligo(dopaminemethacrylamide) homopolymers were ˜110° C. and ˜88° C., respectively.For poly[(dopamine methacrylamide)35%-co-(methyl methacrylate)65%], theTg was at ˜113° C. Using DSC to identify glass transition temperaturesbecame more complicated with PEG chains added to the polymers. The Tg ofa poly[(ethylene glycol) methyl ether methacrylate] homopolymer has beenreported to be −57° C. For all PEG-containing terpolymers here, a quitebroad endothermic peak appeared from ˜−5° C. to ˜75° C. Since a singlepeak was observed by DSC, random arrangement of the monomers in thebackbone was quite likely. In an effort to gain more specific insightson the nature of these polymers, a melting temperature apparatus wasused to locate potential thermal transitions. For poly{[dopaminemethacrylamide]28%-co-[poly(ethylene glycol) methyl ethermethacrylate]72%)}, without methyl methacrylate, the sample was aviscous gel and could not be tested. At 0% PEG, 65% methyl methacrylate,and 35% dopamine methacrylamide, the sample started deforming at ˜140°C. With 52% PEG, 16% methyl methacrylate, and 33% dopaminemethacrylamide, this high PEG sample began deforming at 40° C. Althoughnot precise, these data do show that the incorporation of PEG into thepolymers decreased the glass transition temperatures.

TABLE 1 Composition and molecular weight data for a family ofpoly{[dopamine methacrylamide]-co-[methylmethacrylate]-co-[poly(ethylene glycol) methyl ether methacrylate]}heteropolymers. dopamine metha- methyl poly(ethylene crylamidemethacrylate glycol) M_(n) M_(w) [mole %] [mole %] [mole %] [g mol⁻¹] [gmol⁻¹] PDI 0 100 0 12 200 17 800 1.5 35 65 0  5 900  8 100 1.3 34 58 8 5 300  7 400 1.4 34 54 12  6 100  8 100 1.3 36 41 23 21 000 35 400 1.734 32 34 10 000 14 000 1.4 29 27 45 10 400 20 400 1.9 33 16 52 12 600 21000 1.7 28 0 72 24 900 51 000 2.0

Eight polymers with similar amounts of dopamine methacrylamide (28% to36%) and varying amounts of methyl methacrylate (0% to 65%) versuspoly(ethylene glycol) methyl ether methacrylate (PEG) (0% to 72%) weresynthesized and their lap shear properties tested. No externalcross-linking agents were added. Lap shear is one of the most commonways to evaluate bulk adhesion and was thus used here. Studies on bothstiff (elastic modulus 69 GPa) and flexible (elastic modulus<1 GPa)substrates were conducted to better evaluate the effect of matching thesubstrate and adhesive moduli. Many of our prior studies have bondedaluminum substrates cleaned by the ASTM D2651 standard method.Consequently, lap shear adhesion studies here began with aluminum.Adhesion is defined as the maximum load at failure divided by theglue-covered substrate overlap area.

Past studies with a styrene-based polymer showed that oligo(ethyleneglycol) chains could influence adhesion in some cases. Up to ˜18 molepercent oligo(ethylene glycol)-containing monomer could be copolymerizedwith styrene and 3,4-dihydroxystyrene without seeing any adhesionpenalty, reaching ˜2.5 MPa on polished aluminum. With more than ˜18% ofthe oligo(ethylene glycol) monomer, adhesion diminished significantly,down to ˜0.3 MPa at 35% oligo(ethylene glycol).

For the methacrylate-based polymer used in this disclosure, initialincorporation of PEG resulted in a slight decline in adhesion between 8to 12 monomer percent. As shown in FIG. 2, at loadings of 23% PEG,adhesion started to rise, reaching ˜1.5 MPa. The peak in adhesion at˜2.4 MPa was obtained with 45% PEG in the backbone. At 72%, the highestloading of PEG, adhesion then decreased down to ˜0.2 MPa. Commoncommercial adhesives were tested on aluminum to obtain a comparison ofbonding performances. Under similar conditions, poly(vinyl acetate)Elmer's Glue All adhered at ˜4 MPa, cyanoacrylate Krazy Glue at ˜7 MPa,and Loctite epoxy at ˜11 MPa.

If trying to obtain gains in strength by matching the modulus of theadhesive and the substrate, one might expect that the stiffest substrate(e.g., aluminum in this case) would bond best with the highest modulusadhesive examined. In other words, the 0% PEG polymer, poly[(dopaminemethacrylamide)35%-co-(methyl methacrylate)65%], the left most point inFIG. 2, might be the strongest bonding glue on aluminum. Yet we see thepeak in adhesion at 45% mole percent of the softening PEG monomer.Improving bond strengths does not appear to be as simple as justmatching the modulus of the adhesive and the substrate. Ductility andstrength of the adhesive material must now be considered.

When examining the force versus extension curves from lap shear testingof the adhesive terpolymers, a dramatic effect of PEG became evident(FIG. 3A). These curves are the raw data from adhesion measurementexperiments in which a bonded pair of substrates were pulled untilfailure. Adhesion values reported here use the highest force observedprior to failure. The sharp curves seen for 0%, 11%, 23%, and 34% PEGare indicative of brittle fracture. At loadings of 45% PEG and above,the force versus extension curves became more rounded. The high PEGpolymers appeared to be softening. By incorporating PEG, a brittle toductile transition may have occurred.

For examining potential changes to mechanical properties, the adhesivepolymers were cast into thin films. Dynamic mechanical analysis in thecontrolled force mode was used to obtain stress versus strain data (FIG.3B), the percent elongation of the adhesive polymers being a measurementof the ductility of the polymers. The polymers containing 0% and 23% PEGproved to be incredibly fragile when in a film of ˜0.3 mm thickness. Ifbent at all, the sample would fail catastrophically into numerouspieces. We were only able to obtain estimates of mechanical properties.With more PEG included, the films became increasingly durable, flexible,and workable. The samples containing the most PEG were malleable enoughto be easily folded in half. The elastic moduli of these heteropolymersranged from ˜0.0002 at 72% PEG to ˜2 GPa at 0% PEG (Table 2). Forcomparison, stiff commercial adhesives including epoxies have elasticmoduli in the range of ˜3 to 5 GPa. Flexible adhesives such aspoly(urethanes) are lower at ˜0.1 GPa.

TABLE 2 Mechanical properties for a family of poly{[dopaminemethacrylamide]-co-[methyl methacrylate]-co-[poly(ethylene glycol)methyl ether methacrylate]} heteropolymers. Ultimate dopamine methylpoly(ethylene Elastic Yield tensile methacrylamide methacrylate glycol)modulus stress strength Strain [mole %] [mole %] [mole %] [GPa] [MPa][MPa] [%] 35 65 0 ~2 N/A^(a)) ~3     ~0.2 36 41 23 1.3 ± 0.5 N/A^(a)) 2± 1 0.1 ± 0.1 34 32 34 .015 ± 0.01 5 ± 1 7 ± 1 30 ± 10 29 27 45 0.042 ±0.002 2.2 ± 0.4 4 ± 1 90 ± 30 33 16 52 0.012 ± 0.001  l.l ± 0.l 2.2 ±0.3 110 ± 20  28 0 72 0.00019 ± 0.00003 N/A^(b)) >0.1^(b)) >190^(b))^(a)) Specimens did not yield ^(b)) Specimens did not fail prior toreaching the extension limit of the instrument

When transitioning from the 23% PEG to the 34% PEG polymer, a shift fromelastic to plastic deformation was observed. The stress versus straincurve for the terpolymers containing 34% PEG was no longer linear,displaying a yield point, the stress at which the material began todeform plastically. With this shift came a rise in ductility, whichcould be quantified by the strain at break. With increasing amounts ofPEG, the strain at break rose from ˜0.2% for the 0% PEG polymer to over190% for the 72% PEG polymer. The 72% PEG specimens never actuallyfailed, reaching the extension limit of the instrument at 190%. Althoughthe terpolymer containing 34% PEG displayed the highest mechanicalstrength (˜7 MPa) and intermediate ductility (˜27%), this polymer wasnot the one exhibiting the highest adhesion. The peak adhesion onaluminum was obtained at 45% PEG. This balance between strength (˜4 MPa)and high ductility (˜88%) appears to provide the highest adhesion.

Adhesion tests on substrates of varying stiffness were then conducted todetermine if increases in bonding strengths could be observed when themoduli of both the glue and substrate were chosen to be similar.Poly(urethane) substrates were purchased in a wide range of durometerhardnesses (FIG. 4A). The most flexible substrate, poly(urethane) of 40Shore A hardness, felt similar to a flexible rubber. The stiffestsubstrate, 75 D Shore D poly(urethane), was comparable to a constructionhard hat. With a hardness between these two other substrates, 80 Shore Apoly(urethane) was slightly malleable, similar to that of a shoe heel.Other common plastic substrates tested were poly(methyl methacrylate)and poly(vinyl chloride). Mechanical properties of each material weredetermined by fabricating dumbbell-shaped specimens and testing intension following the ASTM D638 standard method (Table 3). The elasticmoduli of these plastic substrates varied from ˜0.001 to ˜1 GPa. Contactangle measurements showed that the surface energies for the substrateswere generally similar (Table 3).

TABLE 3 Water contact angles and mechanical properties of aluminum andplastic substrates. water elastic ultimate contact modulus yield stresstensile stress angle [°] [GPa] [MPa] [MPa] strain [%] aluminum^([57]) 90± 2 69 276 310 17 poly(methyl 79 ± 3 0.89 ± 0.04 82 ± 2 87 ± 1 12 ± 1methacrylate) poly(vinyl 83 ± 3 0.8 ± 0.1 50 ± 1 40 ± 4 140 ± 60chloride) 75D 81 ± 4 0.5 ± 0.1 N/A^(a)) 27 ± 5 340 ± 70 poly(urethane)80A 85 ± 5 0.016 ± 0.001 N/A^(a)) >6 ± 1 >430 ± 80  poly(urethane) 40A86 ± 7 0.00115 ± 0.00002 N/A^(a))  1.6 ± 0.1 240 ± 20 poly(urethane)^(a))Specimens did not yield.

When bonding together the softer poly(urethane) substrates in lap shearconfigurations, the joints flexed and bent during adhesion testing. Dueto additional stresses on the joints from this bending, results were notconsidered reliable. Subsequently, we explored another simple adhesionconfiguration with butt tensile joints. For comparison to the lap shearresults, testing of all heteropolymers was repeated in the butt tensileconfiguration on aluminum (FIG. 2). These data are overlaid with theanalogous lap shear experiments described earlier (FIG. 2). A roughlysimilar relation of adhesion versus PEG content was observed with bothbutt tensile and lap shear joints. Changes in adhesion relative to PEGcontent were more subtle, however. This observation of a flatter trendin butt tensile versus lap shear may be a result of glues being moreductile in shear than in tension. If a flaw (e.g., void due toevaporation of solvent) exists in a butt tensile joint, once a crack istriggered, the specimen will fail quickly due to this high stressconcentration. In lap shear, if cracking at a void occurs, the remainingadhesive area is still available to deform, by withstanding higherstrains. A consequence here is that butt tensile bonding may not have asmuch to gain from the added PEG ductility as a lap shear joint.

With a bonding testing method in hand, the entire family of moduli-tunedadhesives was tested on the five plastic substrates of varying stiffness(Table 3). Substrates with the highest elastic modulus at ˜1 GPa werepoly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC). Forboth PMMA and PVC substrates, adhesion peaked at 45% PEG, reaching ˜2MPa (FIG. 4B). Adhesion for this 45% PEG terpolymer on these two plasticsubstrates was slightly higher than that for aluminum in butt tensile(˜1.3 MPa) (FIGS. 2 and 4B). The modulus of the 45% PEG terpolymer at˜0.04 GPa (Table 2) is more similar to the moduli of these plastics (˜1GPa) than to that of aluminum at ˜69 GPa. Still, the adhesive is ˜25times less stiff than PMMA and PVC. The rise in adhesion at a point of45% PEG content was likely a result of the adhesive having ductilityfrom the PEG, being able to deform plastically. This ductility andplasticity allows mechanical stresses to be redistributed throughout thematerial while testing occurred.

The most flexible substrate, 40 A poly(urethane) with an elastic modulusof only ˜0.001 GPa (Table 3), displayed the lowest adhesion at ˜0.2 MPafor all terpolymers (FIG. 4B). Due to the flexibility of this substrate,both the adhesive and substrate were being stressed during testing. Thelack of stiffness prohibits a strong adhesive bond. Generally speaking,substrates of low strength and high flexibility are difficult to bondwell, regardless of the adhesive used.

The stiffest poly(urethane) substrate, 75 D, had an elastic modulus of˜0.5 GPa (Table 3). On 75 D poly(urethane), adhesion was highest at 0%PEG and gradually decreased with increasing amounts of this monomeradded to the polymer (FIGS. 4A and 4B). Tensile tests revealed that thissubstrate could be extended to high strains (˜339%) prior to failure(Table 3). Due to this high substrate extensibility, having PEG in theadhesive to promote ductility was not necessary to achieve strongerbonds. All the needed flexibility was already in the substrate.Additional ductility from PEG only served to weaken the system.

The 80 A poly(urethane) substrate had an elastic modulus (˜0.02 GPa)higher than 40 A poly(urethane) (˜0.001 GPa) and lower than 75 Dpoly(urethane) (˜0.45 GPa) (Table 3). A slight rise in adhesion was seenat 52% PEG. At this 52% PEG content, the elastic modulus of the adhesive(˜0.01 GPa) (Table 2) was a close match to that of the substrate. Forthis substrate, the strain at break was so high (>430%) that additionalductility from the adhesive polymer was not required in order to achievemaximum bond strength. Designing the adhesive such that the adhesiveelastic modulus is similar to that of the substrate elastic modulus maybe more important here.

For substrates with higher elastic moduli (≥1 GPa) such as aluminum,PMMA, and PVC, the point of maximum bonding can be found where theadhesive provides ductility, but can also maintain strength (Table 2).Thus for the bonding of metals and commodity plastics, matching theadhesive and substrate moduli is not necessarily the most criticalfactor. Ductility and the resulting decrease in elastic modulus canyield the highest bond strengths (FIG. 2). Too much ductility willweaken the adhesive and bond strengths may suffer. When the elasticmodulus of the substrate is above that of flexible rubber (˜0.001 GPa)or below that of common plastics (˜1 GPa), ductility is often built intothe substrate (Table 3). Adding ductility to the adhesive will notbenefit the joint performance (FIGS. 4A and 4B). For these softersubstrates, the adhesive may work best when the modulus is comparable tothat of the substrate.

Poly(ethylene glycol) is one of the most widely used polymers within thebiomedical industry and also the focus of countless academic studies.Amongst the greatest aspects of this polymer is biocompatibility. In asense, this lack of toxicity has origins in a lack of adhesion. Althoughthe exact reasons behind the biocompatibility of PEG are still debated,the most accepted idea is that water attaches to the polymer's oxygensvia hydrogen bonds. The resulting hydrated structure, in essence, looksjust like water. Macromolecules and cells do not “see” the polymer, donot adhere, and no biological response such as immunogenicity takesplace.

Data presented herein show that the anti-adhesive effects of PEG can bemore complex. High PEG content within the polymer increased ductility,weakened the material strength, and could decrease adhesion. However,moderate levels of PEG actually increased adhesion. When considering theanti-adhesive or anti-fouling effects of PEG, we should likely keep inmind which mechanisms are most relevant to the situation at hand.

Prior studies examining adhesive modulus or ductility are few, with theavailable data being derived from, effectively, mixing fillers intocommercial adhesives. Here, we varied the composition of aheteropolymer, without adding external fillers, flexibilizers,plasticizers or cross-linkers. This approach also avoided phaseseparations, such as those observed with rubber-toughened epoxies.Tuning the amount of methyl methacrylate and PEG in the polymer providedadhesives with moduli ranging from ˜0.0002-2 GPa, strengths from ˜0.1-3MPa, and strains from ˜0.2-200%. By synthesizing these heteropolymers,there was more control over the structure, allowing a wider range ofmechanical properties when compared to prior studies. This type of asystematic method allowed us to identify the point at which strength andductility were complementary, leading to a toughened adhesive.

These data also shed light on some general design principles for makingglues. Addition of poly(ethylene glycol) into the polymer chains allowedidentification of the point at which both strength and ductility werebalanced. Where adhesion peaked was also quite dependent upon the natureof the substrate being bonded. Although matching the moduli of adhesivesto substrates should be considered, tuning the adhesive ductility is atleast of equivalent importance. Further insights were provided on theanti-adhesive or anti-fouling aspect of PEG. Increased ductility anddecreased material strength with PEG can influence adhesionsignificantly, but by a mechanism quite different than that found formaterials placed in biological or aqueous contexts.

General Procedures:

A Varian Inova-300 MHz spectrometer was used to record proton nuclearmagnetic resonance (¹H NMR) spectra. In order to integrate the peaksaccurately, a relaxation delay of 30 s between scans was implemented.Monomer ratios in the final polymers were determined by integration ofthe aromatic region (δ: 6.2-6.7 ppm) to give dopamine methacrylamidecontent, the —OCH₂ peak at 3.8-4.2 ppm for poly(ethylene glycol methylether methacrylate) content, and the backbone region (δ: 0-2.3 ppm) formethyl methacrylate content.

Molecular weights were found by gel permeation chromatography (GPC)using a Polymer Laboratories PL-GPC20 with eluent THF. Water contactangles for all substrates were determined using a Rame-Hart AdvancedGoniometer/Tensiometer Model 500. Thermal transitions were observed witha Perkin Elmer Jade Differential Scanning Calorimeter (DSC) from −40° C.to 140° C. at 5° C. min⁻¹.

Methyl methacrylate and poly(ethylene glycol) methyl ether methacrylate(Mn-300 g mol⁻¹) monomers were purchased from Sigma Aldrich and purifiedusing an alumina column. This Mn translates to an oligo(ethylene glycol)(OEG) chain length of ˜4.3 ethylene glycol repeats. Synthesis of thedopamine methacrylamide monomer followed a published procedure andcharacterization employed 1H NMR spectroscopy, as shown in FIG. 5. Allpolymers were prepared by free radical polymerization under an inertargon atmosphere using typical Schlenk techniques. The radicalinitiator, azobisisobutyronitrile (AIBN), was recrystallized frommethanol and dried in vacuo prior to use. DMF solvent was kept oversieves and degassed with bubbling argon for at least 15 minutes prior tostarting a reaction. To synthesize a family of terpolymers with targetmonomer compositions, the ratio of methyl methacrylate to poly(ethyleneglycol) methyl ether methacrylate was altered in the feed. The contentof dopamine methacrylamide in the polymer was always targeted to be 33mole %.

Combining a radical initiator such as AIBN with a radical inhibitor suchas a catechol compound may appear to be counterintuitive. Severalresearch groups have been producing acrylate polymers, often containingdopamine methacrylamide, using the general synthetic methods describedbelow. This class of polymer can contain varied degrees of cross-linkingat the end of the synthesis. From this report and our target of ˜33%dopamine methacrylamide, we may surmise that the polymers describedbelow contain roughly one cross-link for every third polymer. In otherwords, the degree of cross-linking in the materials synthesized for thiscurrent disclosure is quite low. Prior to adhesion studies, the majorityof isolated polymer chains are free of any cross-links.

Synthesis of poly([dopamine methacrylamide]-co-[methylmethacrylate]-co-[poly(ethylene glycol) methyl ether methacrylate])

Dopamine methacrylamide (1.5 g, 6.7 mmol), methyl methacrylate (0.68 mL,6.4 mmol), poly(ethylene glycol) methyl ether methacrylate (1.9 mL, 6.5mmol), and AIBN (31.6 mg, 0.192 mmol) were dissolved intodimethylformamide (14 mL) in a flame dried Schlenk flask. After stirringfor 30 minutes under argon and at room temperature, the flask was placedinto an 80° C. oil bath for 2 days. The reaction mixture became aviscous solution. The flask was removed from the oil bath and 1 mL ofmethanol was added to quench the reaction. To the cooled reaction wasadded dichloromethane (˜10 mL) for dilution. The solution was thenpoured into excess ether (˜200 mL) to precipitate a white polymer. Theproduct was reprecipitated two additional times indichloromethane/ether. Sonication along with minimal methanol was oftennecessary to solubilize the polymer. The product was dried in vacuo fortwo nights yielding 3.3 g (81%) of pure polymer.

Synthesis of poly([dopamine methacrylamide]-co-[poly(ethylene glycol)methyl ether methacrylate])

Dopamine methacrylamide (0.89 g, 4.1 mmol), poly(ethylene glycol) methylether methacrylate (2.2 mL, 7.5 mmol), and AIBN (18.7 mg, 0.14 mmol)were dissolved into dimethylformamide (9 mL) in a flame dried Schlenkflask. After stirring for 30 minutes under argon and at roomtemperature, the flask was placed in a 65° C. oil bath for 5.5 hours. Atthis point, the reaction mixture was starting to gel. The flask wasremoved from the oil bath and 1 mL of methanol was added to quench thereaction. The reaction mixture was poured into excess ether (150 mL) toprecipitate a white polymer. The product was reprecipitated twoadditional times in dichloromethane/ether and then dried in vacuo fortwo nights.

Synthesis of poly[(dopamine methacrylamide)-co-(methyl methacrylate)]

Dopamine methacrylamide (1.1 g, 5.1 mmol), methyl methacrylate (1.0 mL,9.5 mmol), and AIBN (23.6 mg, 0.14 mmol) were dissolved intodimethylformamide (7 mL) in a flame dried Schlenk flask. After stirringfor 30 minutes under argon at room temperature, the flask was placed ina 70° C. oil bath for 17 hours. The reaction mixture became a viscoussolution. Upon removal from the oil bath, 1 mL of methanol was added. Tothe cooled reaction was added ˜5 mL of dichloromethane for dilution.This solution was poured into excess ether (˜200 mL) to precipitate abrownish-white polymer. The product was reprecipitated two additionaltimes in dichloromethane/ether and then dried in vacuo for two nights.

Synthesis of Poly(Methyl Methacrylate):

Methyl methacrylate (2.45 mL, 22.9 mmol) and AIBN (37 mg, 0.23 mmol)were added to anhydrous toluene (15 mL) in a flame dried Schlenk flask.After stirring for 30 minutes at room temperature under argon, the flaskwas placed into a 75° C. oil bath overnight. The flask was removed fromthe oil bath and 1 mL of methanol was added. This reaction mixture waspoured into excess hexanes (150 mL) to precipitate a white polymer. Theproduct was reprecipitated two additional times indichloromethane/hexanes and then dried in vacuo for two nights.

Mechanical Properties:

Stress versus strain curves were obtained for the DOPA-containingterpolymers using dynamic mechanical analysis (TA Instruments Q800) intension with controlled force (0.10 N minute⁻¹). A preload force of0.001 N was applied. Thin film samples were prepared by solvent casting.Typically, 0.05 g of polymer was dissolved at 0.15 g mL⁻¹ in 10% volumetrichloroethylene in methanol. Using a micropipette, the polymersolution (50 μL) was added into a custom fashioned polydimethylsiloxanemold every 30 minutes until the entire sample was formed. Samples werecured at room temperature for ˜6 hours, then at 37° C. for 48 hours.Typical samples were 12.0 mm long, 3.0 mm wide, and 0.3 mm thick. Toclamp into the instrument with consistent forces, a torque wrench wasused. When samples had a PEG content greater than 23%, a torque of 2in-lb was applied. For lower percent PEG samples, films could only besecured at fingertip tightness without breaking. At least three sampleswere tested for each trial. For the 0% PEG polymer, only one trial isreported due to how fragile the thin films were. Testing of at least 10samples were attempted, but could not be secured into the grips withoutcracking. Due to the flexibility and extensibility of 72% PEG, thesesamples did not fail before reaching the extension limit of theinstrument.

Tensile properties of the substrates were determined by the ASTM D638standard method. Specimens were fabricated into Type IV dumbbell shapesusing a CNC mill for 75 D polyurethane, poly(methyl methacrylate), andpoly(vinyl chloride). A water jet cutter was used for the 40 A and 80 Apolyurethane substrates. Specimens were tested on an MTS InsightElectromechanical Testing System using a 2000 N load cell.Poly(urethane) specimens were tested at 50 mm min⁻¹. The poly(methylmethacrylate) and poly(vinyl chloride) substrates were tested at 5 mmmin⁻¹. It should be noted that 80 A poly(urethane) slipped out of thegrips at high extensions for all samples due to decreasingcross-sectional area as testing.

For both adhesive films and substrates, the elastic moduli weredetermined from the initial slope of the linear portion of thestress-strain curve. For 75 D poly(urethane) specimens the initial slopewas not linear, thus the secant modulus is reported. The yield strengthwas defined by the 0.2% offset strain. Ultimate tensile strength was theload at fracture divided by the area. Strain was the percent elongationat break. Averages and errors at +1 standard deviation are reported.

For lap shear tests using aluminum substrates, adherends of 8.89 cm×1.27cm×0.318 cm were precision cut using a water jet system from a sheet ofaluminum 6061-T6 purchased at Farmer's Copper. For butt tensile tests,aluminum 6061-T6 rod stock with a diameter of 1.59 cm was cut and facedto a length of 7.68 cm using a CNC mill. Holes with a diameter of 0.633cm were drilled 0.80 cm from the top using a drill press. The adherendswere cleaned according to the ASTM D2651 standard method, followed bywashes in boiling, deionized water and methanol.

For lap shear and butt tensile tests using plastic substrates, toluenediisocyanate based poly(urethane) bar stock (1.27 cm×1.27 cm×61.0 cm)and rod stock (1.59 cm×122 cm) with durometer hardnesses of 40 Shore A,80 Shore A, and 75 Shore D were purchased from Universal UrethaneProducts. Using a mold, specimens were cut with a Walnut HollowProfessional Hot Knife to a length of 8.89 cm for lap shear and 7.68 cmfor butt tensile. Additionally for butt tensile tests, cast acrylic(i.e., PMMA) and polyvinyl chloride (Type II) rod stock with a diameterof 1.59 cm were purchased from McMaster Carr. Specimens were cut with aband saw to 8.0 cm and then a CNC lathe was used to face the ends givinga length of 7.68 cm. Using a drill press, holes with a diameter of 0.633cm were drilled 0.80 cm from the top for 75 D poly(urethane), castacrylic, and polyvinyl chloride substrates. To clean prior to testing,soap and water were used, followed by ethanol and hexane washes. Thespecimens were heated at 70° C. to dry for 2 hours.

For adhesion tests, the polymers were dissolved at 0.15 g polymer mL-1methanol, often using a sonicator. At high methyl methacrylatepercentages (˜41-65 mole %), the solvent used was 8% volumetrichloroethylene in methanol due to solubility issues. Polymersolutions (45 μL) were deposited onto the adherends and then overlapped(1.2×1.2 cm) to form single lap-joint configurations. The use of ahomebuilt jig ensured consistency of the overlap area and alignment ofthe joints. Two Teflon blocks on either side of the joint were pushedtogether to precisely align the bonded substrates. For butt tensilespecimens, after applying the polymer solutions (45 μL), 60 μm soda limeborosilicate glass beads from McMaster Carr were added beforeoverlapping in order to control the thickness. A separate jig was usedto ensure proper alignment of the butt tensile substrates onceoverlapped. Specimens were allowed to cure for 1 hour at roomtemperature followed by 22 hours at 70° C. and then 1 hour at roomtemperature before testing. No cross-linker was added to the polymer,allowing the adhesive to cure into a transparent film with a slightbrown tint.

Single lap joint specimens were tested following a modified version ofthe ASTM D1002 standard. Butt tensile specimens were measured by theASTM D2095 standard method. A modified version of ASTM D2095 wasfollowed when testing the 40 A and 80 A poly(urethane) substrates.Vacuum hose clamps were used to secure these substrates in place. Alltrials were tested on an Instron 5544 Materials Testing System with a2000 N load cell. A crosshead speed of 2 mm min⁻¹ was used. For eachrun, a data set of at least five samples was collected. Averages anderrors at 95% confidence intervals are reported. In all cases, testedbonds showed evidence of cohesive failure, with roughly even amounts ofpolymer left on each substrate after bond breakage.

It should be appreciated that other materials can be used in place ofthose described herein, which were meant for demonstrative purposes. Forexample, adding PEG into other polymers also has been seen to helpadhesion, which is theorized to be due to a similar mechanism ofincreasing ductility. The methods and polymers described can be appliedto designs of adhesives in other non-acrylate systems as well, and istherefore applicable to a broader class of materials.

Various aspects of different embodiments of the present invention areexpressed in paragraphs X1, X2, X3 and X4 as follows:

X1. One aspect of the present invention pertains to an adhesivecomposition comprising a heteropolymer including one of dopaminemethacrylamide or 3,4-dihydroxyphenylalanine or 3,4-dihydroxystyrene,and at least one of methyl methacrylate, styrene, and poly(ethyleneglycol) methyl ether methacrylate.

X2. Another aspect of the present invention pertains to an adhesivecomposition comprising a heteropolymer including a plurality of monomersincluding a dopamine moiety and a plurality of monomers including anacrylate moiety.

X3. A further aspect of the present invention pertains to a biomimeticpolymer adhesive comprising the following components: cross-linkingmonomer in a proportion of about 28% to about 36% by mole percentage,stiffening monomer in a proportion of 0% to about 65% by molepercentage, and softening monomer in a proportion of about 0% to about72% by mole percentage.

X4. A certain aspect of the present invention pertains to a method ofadhering, the method comprising selecting a pair of substrates to beadhered, determining an elastic modulus of each substrate, and adheringthe substrates using a heteropolymer adhesive including a plurality ofcross-linking monomers and a plurality of stiffening monomers, wherein,if the elastic modulus of each of the substrates is greater than about 1GPa, the heteropolymer adhesive further includes a plurality ofsoftening monomers.

Yet other embodiments pertain to any of the previous statements X1, X2,X3 or X4 which are combined with one or more of the following otheraspects.

Wherein the one of dopamine methacrylamide or 3,4-dihydroxyphenylalanineor 3,4-dihydroxystyrene is present in a proportion of about 10% to about50% by mole percentage.

Wherein the one of dopamine methacrylamide or 3,4-dihydroxyphenylalanineor 3,4-dihydroxystyrene is present in a proportion of about 20% to about40% by mole percentage.

Wherein the one of dopamine methacrylamide or 3,4-dihydroxyphenylalanineor 3,4-dihydroxystyrene is present in a proportion of about 28% to about36% by mole percentage.

Wherein the one of dopamine methacrylamide or 3,4-dihydroxyphenylalanineor 3,4-dihydroxystyrene is present in a proportion of about 33% by molepercentage.

Wherein the heteropolymer is a terpolymer including one of dopaminemethacrylamide or 3,4-dihydroxyphenylalanine or 3,4-dihydroxystyrene;methyl methacrylate or styrene; and poly(ethylene glycol) methyl ethermethacrylate.

Wherein the heteropolymer is a terpolymer including dopaminemethacrylamide, methyl methacrylate and poly(ethylene glycol) methylether methacrylate.

Wherein the heteropolymer includes dopamine methacrylamide and at leastone of methyl methacrylate and poly(ethylene glycol) methyl ethermethacrylate.

Wherein the heteropolymer includes 3,4-dihydroxyphenylalanine and atleast one of methyl methacrylate and poly(ethylene glycol) methyl ethermethacrylate.

Wherein the monomers including the dopamine moiety are dopaminemethacrylamide.

Wherein the plurality of monomers including the acrylate moiety are aplurality of monomers including a methacrylate moiety.

Wherein the monomers including the methacrylate moiety are methylmethacrylate.

Wherein the monomers including the methacrylate moiety are poly(ethyleneglycol) methyl ether methacrylate.

Wherein the monomers including the methacrylate moiety are methylmethacrylate and poly(ethylene glycol) methyl ether methacrylate.

Wherein at least one of the stiffening monomer and the softening monomeris a methacrylate monomer.

Wherein the stiffening monomer is present in a non-zero amount andwherein the softening monomer is present in a non-zero amount.

Wherein the stiffening monomer is methyl methacrylate.

Wherein the softening monomer is poly(ethylene glycol) methyl ethermethacrylate.

Wherein the cross-linking monomer is one of dopamine methacrylamide or3,4-dihydroxyphenyl alanine.

Wherein the stiffening monomer is present in a proportion of about 16%to about 58% by mole percentage and wherein the softening monomer ispresent in a proportion of about 8% to about 52% by mole percentage.

Wherein the stiffening monomer is present in a proportion of about 16%to about 41% by mole percentage and wherein the softening monomer ispresent in a proportion of about 23% to about 52% by mole percentage.

Wherein the cross-linking monomers include a dopamine moiety.

Wherein the cross-linking monomers are one of dopamine methacrylamide,3,4-dihydroxyphenylalanine, and 3,4-dihydroxystyrene.

Wherein the softening monomers include a poly(ethylene glycol) moiety.

Wherein at least one of the stiffening monomers and the softeningmonomers includes an acrylate moiety.

Wherein at least one of the stiffening monomers and the softeningmonomers includes a methacrylate moiety.

Wherein the stiffening monomers are methyl methacrylate.

Wherein the softening monomers are poly(ethylene glycol) methyl ethermethacrylate.

Wherein the cross-linking monomers are present in a proportion of about10% to about 50% by mole percentage.

Wherein the cross-linking monomers are present in a proportion of about20% to about 40% by mole percentage.

Wherein the cross-linking monomers are present in a proportion of about28% to about 36% by mole percentage.

Wherein the plurality of cross-linking monomers and the plurality ofstiffening monomers are present in proportions resulting in an elasticmodulus of the heteropolymer adhesive substantially equal to the elasticmodulus of at least one of the pair of substrates.

Wherein the plurality of cross-linking monomers and the plurality ofstiffening monomers are present in proportions resulting in an elasticmodulus of the heteropolymer adhesive less than 25× greater than theelastic modulus of at least one of the pair of substrates, and less than25× lower than the elastic modulus of at least one of the pair ofsubstrates.

Wherein the plurality of cross-linking monomers and the plurality ofstiffening monomers are present in proportions resulting in an elasticmodulus of the heteropolymer adhesive less than 10× greater than theelastic modulus of at least one of the pair of substrates, and less than10× lower than the elastic modulus of at least one of the pair ofsubstrates.

Wherein the plurality of cross-linking monomers and the plurality ofstiffening monomers are present in proportions resulting in an elasticmodulus of the heteropolymer adhesive less than 5× greater than theelastic modulus of at least one of the pair of substrates, and less than5× lower than the elastic modulus of at least one of the pair ofsubstrates.

The foregoing detailed description is given primarily for clearness ofunderstanding and no unnecessary limitations are to be understoodtherefrom for modifications can be made by those skilled in the art uponreading this disclosure and may be made without departing from thespirit of the invention.

What is claimed is:
 1. An adhesive composition comprising aheteropolymer including one of dopamine methacrylamide or3,4-dihydroxyphenylalanine or 3,4-dihydroxystyrene, and at least one ofmethyl methacrylate, styrene, and poly(ethylene glycol) methyl ethermethacrylate.
 2. The adhesive composition of claim 1, wherein the one ofdopamine methacrylamide or 3,4-dihydroxyphenylalanine or3,4-dihydroxystyrene is present in a proportion of about 10% to about50% by mole percentage.
 3. The adhesive composition of claim 2, whereinthe one of dopamine methacrylamide or 3,4-dihydroxyphenylalanine or3,4-dihydroxystyrene is present in a proportion of about 20% to about40% by mole percentage.
 4. The adhesive composition of claim 3, whereinthe one of dopamine methacrylamide or 3,4-dihydroxyphenylalanine or3,4-dihydroxystyrene is present in a proportion of about 28% to about36% by mole percentage.
 5. The adhesive composition of claim 1, whereinthe heteropolymer is a terpolymer including one of dopaminemethacrylamide or 3,4-dihydroxyphenylalanine or 3,4-dihydroxystyrene;methyl methacrylate or styrene; and poly(ethylene glycol) methyl ethermethacrylate.
 6. The adhesive composition of claim 1, wherein theheteropolymer includes dopamine methacrylamide and at least one ofmethyl methacrylate and poly(ethylene glycol) methyl ether methacrylate.7. The adhesive composition of claim 1, wherein the heteropolymerincludes 3,4-dihydroxyphenylalanine and at least one of methylmethacrylate and poly(ethylene glycol) methyl ether methacrylate.
 8. Anadhesive composition comprising a heteropolymer including a plurality ofmonomers including a dopamine moiety and a plurality of monomersincluding an acrylate moiety.
 9. The adhesive composition of claim 8,wherein the monomers including the dopamine moiety are dopaminemethacrylamide.
 10. The adhesive composition of claim 8, wherein theplurality of monomers including the acrylate moiety are a plurality ofmonomers including a methacrylate moiety.
 11. The adhesive compositionof claim 10, wherein the monomers including the methacrylate moiety aremethyl methacrylate.
 12. The adhesive composition of claim 10, whereinthe monomers including the methacrylate moiety are poly(ethylene glycol)methyl ether methacrylate.
 13. The adhesive composition of claim 10,wherein the monomers including the methacrylate moiety are methylmethacrylate and poly(ethylene glycol) methyl ether methacrylate.
 14. Abiomimetic polymer adhesive comprising the following components:cross-linking monomer in a proportion of about 28% to about 36% by molepercentage, stiffening monomer in a proportion of 0% to about 65% bymole percentage, and softening monomer in a proportion of about 0% toabout 72% by mole percentage.
 15. The adhesive of claim 14, wherein atleast one of the stiffening monomer and the softening monomer is amethacrylate monomer.
 16. The adhesive of claim 14, wherein thestiffening monomer is present in a non-zero amount and wherein thesoftening monomer is present in a non-zero amount.
 17. The adhesive ofclaim 14, wherein the stiffening monomer is methyl methacrylate.
 18. Theadhesive of claim 14, wherein the softening monomer is poly(ethyleneglycol) methyl ether methacrylate.
 19. The adhesive of claim 14, whereinthe cross-linking monomer is one of dopamine methacrylamide or3,4-dihydroxyphenylalanine.
 20. The adhesive of claim 14, wherein thestiffening monomer is present in a proportion of about 16% to about 41%by mole percentage and wherein the softening monomer is present in aproportion of about 23% to about 52% by mole percentage.
 21. A method ofadhering, the method comprising: selecting a pair of substrates to beadhered; determining an elastic modulus of each substrate; and adheringthe substrates using a heteropolymer adhesive including a plurality ofcross-linking monomers and a plurality of stiffening monomers; wherein,if the elastic modulus of each of the substrates is greater than about 1GPa, the heteropolymer adhesive further includes a plurality ofsoftening monomers.
 22. The method of claim 21, wherein thecross-linking monomers include a dopamine moiety.
 23. The method ofclaim 21, wherein the cross-linking monomers are one of dopaminemethacrylamide, 3,4-dihydroxyphenylalanine, and 3,4-dihydroxystyrene.24. The method of claim 21, wherein the softening monomers include apoly(ethylene glycol) moiety.
 25. The method of claim 21, wherein atleast one of the stiffening monomers and the softening monomers includesan acrylate moiety.
 26. The method of claim 25, wherein at least one ofthe stiffening monomers and the softening monomers includes amethacrylate moiety.
 27. The method of claim 26, wherein the stiffeningmonomers are methyl methacrylate.
 28. The method of claim 26, whereinthe softening monomers are poly(ethylene glycol) methyl ethermethacrylate.
 29. The method of claim 21, wherein the cross-linkingmonomers are present in a proportion of about 10% to about 50% by molepercentage.
 30. The method of claim 29, wherein the cross-linkingmonomers are present in a proportion of about 20% to about 40% by molepercentage.
 31. The method of claim 21, wherein the plurality ofcross-linking monomers and the plurality of stiffening monomers arepresent in proportions resulting in an elastic modulus of theheteropolymer adhesive substantially equal to the elastic modulus of atleast one of the pair of substrates.
 32. The method of claim 21, whereinthe plurality of cross-linking monomers and the plurality of stiffeningmonomers are present in proportions resulting in an elastic modulus ofthe heteropolymer adhesive less than 25× greater than the elasticmodulus of at least one of the pair of substrates, and less than 25×lower than the elastic modulus of at least one of the pair ofsubstrates.
 33. The method of claim 21, wherein the plurality ofcross-linking monomers and the plurality of stiffening monomers arepresent in proportions resulting in an elastic modulus of theheteropolymer adhesive less than 10× greater than the elastic modulus ofat least one of the pair of substrates, and less than 10× lower than theelastic modulus of at least one of the pair of substrates.